1. Field of the Invention
The present invention generally relates to an image forming apparatus including a transferer and methods of setting a transfer electrical current and forming an image.
2. Discussion of the Background
In general, an electrophotographic image forming apparatus employing an intermediate transfer method, such as a copying machine, a printer, a facsimile machine, etc., includes a photoreceptor, an intermediate transfer member and transferers of a primary transferer and a secondary transferer. Such an image forming apparatus forms a toner image on the photoreceptor according to image information, and transfers the toner image with the primary transferer onto the intermediate transfer member, for example, an endless intermediate transfer belt stretched around a plurality of rollers. The primary transferer receives a transfer bias (transfer voltage) to form a transfer electrical field between the photoreceptor and the intermediate transfer belt. The toner image is further transferred onto a transfer material by the secondary transferee. The primary and the secondary transferers preferably transfer the toner image precisely and reliably onto the intermediate transfer belt or the transfer material. That is, high reliability and transfer efficiency are required.
As a method of controlling the transfer bias, a constant-current control method has been proposed. Because electric resistance of the transferee changes with changes in environmental conditions (e.g., temperature and humidity) and energization conditions, a voltage applied to the transferer is adjusted in response to the changes. Therefore, a stable electric field and high transfer efficiency can be obtained.
However, the resistance of the transferer further changes over time by receiving the transfer voltage. When the resistance of the transferer is extremely low, effects of the resistance of a toner layer increases. Therefore, transfer efficiency is changed depending on the area of the toner image. When the resistance of the transferer is extremely high, a higher voltage is applied to the transferee, which may cause a leak current and accordingly lower transfer efficiency. Further, transfer efficiency changes depending on the size of the transfer material in the case of the secondary transferee, although an image forming apparatus is generally configured to handle different sizes of transfer materials.
To cope with the change over time in the resistance of the transferer, a method in which a transfer voltage is controlled based on a detected resistance value of the transferer has been proposed. Further, to cope with changes in the resistance due to changes in environmental conditions, a method using an environment detector and a table storing current adjustment values in relation to environmental conditions has been proposed. The environment detector is configured to detect environmental conditions while image forming is performed. The transfer voltage is controlled by correcting a current output to the transferer based on a detected condition and correlation in the table.